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36
A nalog Processing
Op tim izing offset p erform ance and gain in the analog d om ain allow s you to achieve a better signal-to-
noise ratio and d ynam ic range than you w ou ld achieve by trying to op tim ize the offset in the d igital
d om ain. As a resu lt, p erform all analog ad ju stm ents p rior to any d igital ad ju stm ents.
Analog gain is m u ltip lied by the analog signal to increase the signal strength before the A/ D conversion.
It is u sed to take ad vantage of the fu ll d ynam ic range of the A/ D converter. For exam p le, in a low light
situ ation the brightest p art of the im age m ay be consistently com ing in at onl y 50% of the DN . An analog
gain of 6 d B (2x) w ill ensu re fu ll u se of the d ynam ic range of the A/ D converter. Of cou rse the noise is
also increased . N ote: To m aintain valid LUT calibration, u se the
Digital Processing
To op tim ize cam era p erform ance, d igital signal p rocessing shou ld be com p leted after any analog
ad ju stm ents.
Fixed p attern noise (FPN ) calibration (calcu lated u sing the
ind ivid u al p ixel d ark cu rrent.
Photo-Resp onse N on-Uniform ity (PRN U) coefficients (calcu lated u sing the
correct the d ifference in resp onsivity of ind ivid u al p ixels (i.e. given the sam e am ou nt of light d ifferent
p ixels w ill charge u p at d ifferent rates) and the change in light intensity across the im age either becau se
of the light sou rce or d u e to op tical aberrations (e.g. there m ay be m ore light in the ce nter of the im age).
PRN U coefficients are m u ltip liers and are d efined to be of a valu e greater than or equ al to 1. This ensu res
that all p ixels w ill satu rate together.
ssb
Backgrou nd su btract (
im age contrast after FPN and PRN U calibration. It is u sefu l for system s that p rocess 8-bit d ata bu t w ant
to take ad vantage of the cam era's 12 bit d igital p rocessing chain. For exam p le, if you find that you r im age
is consistently betw een 128 and 255 DN (8 bit), you can su btract off 128 (
ssg 0 8192
2 (
) to get an ou tp u t range from 0 to 255 DN .
Calibrating the Camera to Remove Non-Uniformity (Flat Field
Correction)
Flat Field Correction Overview
This cam era has the ability to calcu late correction coefficients in ord er to rem ove non -u niform ity in the
im age. This vid eo correction op erates on a p ixel-by-p ixel basis and im p lem ents a tw o-p oint correction for
each p ixel. This correction can red u ce or elim inate im age d istortion cau sed by the follow ing factors:
Fixed Pattern N oise (FPN )
Photo Resp onse N on Uniform ity (PRN U)
Lens and light sou rce non -u niform ity
Correction is im p lem ented su ch that for each p ixel:
- FPN ( p ixel ) - black level offset) * PRN U(p ixel) – Backgrou nd Su btract] x System Gain
V
=[(V
output
input
w here
V
output
V
input
PRN U( p ixel)
FPN ( p ixel )
Backgrou nd
03-032-20116-01
com m and ) and system (d igital) gain (
=
d igital ou tp u t p ixel valu e
=
d igital inp u t p ixel valu e from the CCD
=
PRN U correction coefficient for this p ixel
=
FPN correction coefficient for this p ixel
=
backgrou nd su btract valu e
Spyder3 SC-34 Color Camera User's Manual
ssg
com m and .
ccf
com m and ) is u sed to su btract aw ay
cpa
com m and ) are u sed to
ssg
com m and ) are u sed to increase
ssb 2048
) and then m u ltip ly by
Teledyne DALSA
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